linux-kernel - Re: [PATCH 3/4] thp: fix split vs. unmap race

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Date:	Thu, 5 Nov 2015 11:26:22 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Hillf Danton <hillf.zj@...baba-inc.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	'Minchan Kim' <minchan@...nel.org>
Subject: Re: [PATCH 3/4] thp: fix split vs. unmap race

On Wed, Nov 04, 2015 at 05:20:15PM +0800, Hillf Danton wrote:
> > @@ -1135,20 +1135,12 @@ void do_page_add_anon_rmap(struct page *page,
> >  	bool compound = flags & RMAP_COMPOUND;
> >  	bool first;
> > 
> > -	if (PageTransCompound(page)) {
> > +	if (compound) {
> > +		atomic_t *mapcount;
> >  		VM_BUG_ON_PAGE(!PageLocked(page), page);
> > -		if (compound) {
> > -			atomic_t *mapcount;
> > -
> > -			VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> > -			mapcount = compound_mapcount_ptr(page);
> > -			first = atomic_inc_and_test(mapcount);
> > -		} else {
> > -			/* Anon THP always mapped first with PMD */
> > -			first = 0;
> > -			VM_BUG_ON_PAGE(!page_mapcount(page), page);
> > -			atomic_inc(&page->_mapcount);
> > -		}
> > +		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> > +		mapcount = compound_mapcount_ptr(page);
> > +		first = atomic_inc_and_test(mapcount);
> >  	} else {
> >  		VM_BUG_ON_PAGE(compound, page);
> 
> Then this debug info is no longer needed.
> >  		first = atomic_inc_and_test(&page->_mapcount);

Right.

diff --git a/mm/rmap.c b/mm/rmap.c
index 0837487d3737..a9550b1f74cd 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1186,7 +1186,6 @@ void do_page_add_anon_rmap(struct page *page,
 		mapcount = compound_mapcount_ptr(page);
 		first = atomic_inc_and_test(mapcount);
 	} else {
-		VM_BUG_ON_PAGE(compound, page);
 		first = atomic_inc_and_test(&page->_mapcount);
 	}
 
-- 
 Kirill A. Shutemov
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